A Dunedin MP is accusing Kiwirail of manufacturing figures to send jobs offshore.

Kiwirail made 44 workers redundant at its Hillside workshops near Dunedin last year, after estimating it would cost 25-30 per cent more to build new train wagons in New Zealand than it would to build them in China.

However, a briefing document to Kiwirail's board in August said the cost to build the 300 flat-top freight wagons onshore would actually be 50 per cent more than the Chinese bid of an estimated $29 million, Radio New Zealand reports.

The Kiwirail briefing says Hillside's bid for the work was based on a "very optimistic view" of costs and productivity.

Kiwirail chief executive Jim Quinn says that bid was much more than the company could afford, so it went with the bid it could afford.

But Labour MP Clare Curran is disputing Kiwirail's figures.

"I think [the briefing document] has been written deliberately to make it look as if the cost would be higher than it actually would be," she said.

Ms Curran says there's been a "whitewash" of the issue and wants an independent evaluation of the costs.

The Rail and Maritime Transport Union says Hillside's price was not as high as either of Kiwirail's estimates.